1. Field of the Invention
The invention relates to a data conversion apparatus and an encoding apparatus which are useful in a magnetic recording/reproduction apparatus such as a digital VTR for recording and reproducing a digital signal, and particularly to a data conversion apparatus and an encoding apparatus which are suitable for a high density recording.
2. Description of Related Art
FIG. 1 shows a diagram illustrating an ATF (Automatic Track Finding) servo of the 2-frequency pilot system. In FIG. 1, the reference numeral 70 denotes a magnetic tape. Formed in the magnetic tape 70, are A track 71, which has plus azimuth and on which a pilot signal f1 is recorded, B track 72, which has minus azimuth, and A track 73, which has plus azimuth and on which a pilot signal f2 is recorded. Numeral 74 denotes B head which has minus azimuth.
Prior art data conversion techniques include the 8-10 conversion which is shown in, for example, Japanese Patent Application Laid-Open No. 1-317280(1989). The 8-10 conversion is a method in which, prior to recording digital data into information tracks of a recording medium, continuous data words (applied data words) of an applied digital signal are converted into channel words in the form of selected channel codes. The channel codes have different CDSs.
FIG. 2 diagrammatically shows an example of a table having three channel word strings (C1, C2, C3). One string is assigned with 256 (=2.sup.8) data words which represent numerals from 0 to 255 in decimal notation. For each data word I(i) , each string includes a channel word Cj(i) where j =1, 2 or 3. These data words have different CDSs. Therefore, three channel words can be utilized for one data word I(i).
The 8-10 conversion includes a second signal which relates to a recorded information signal and has a carrier wave of a relatively low frequency. This signal is a pilot signal which serves as a tracking signal giving information on the relative position of a read-out element with respect to an information track, namely the track crossing position.
Systems of controlling tracking using a pilot signal include the ATF servo shown in FIG. 1. The B head 74 is slightly wider than the B track 72, and reproduces a signal on the B track 72 and cross-talk signals from the A tracks 71 and 73 on both sides. When tracking is correctly performed, the cross-talk signals have the same reproduction level. By using this fact, the ATF servo performs a tracking control in which the reproduced signal is passed through a BPF of a pass bandwidth frequency of f1 and a BPF of a pass bandwidth frequency of f2 to extract pilot signals of f1 and f2, the extracted pilot signals are integrated, and the tracking is adjusted so that the reproduction levels through f1 and f2 become the same.
According to the 8-10 conversion, in order to insert the pilot signals of f1 and f2 into the data stream of channel words, a control signal corresponding to the second signal is generated, and channel words are selected from the table of FIG. 2 so that the mean value of the recorded digital signals changes in a manner approximately coincident with the change of the second signal of a relatively low frequency.
At present, a digital VTR which can record images for a long time with high image quality and which uses a miniature cassette is under development. For such a digital VTR, a high density magnetic recording/reproduction technique is indispensable. An example of a high density magnetic recording/reproduction technique is a narrow-track technique. Hereinafter, this technique will be described.
When a pilot signal is added to a record signal, there appears an influence that the error rate is impaired in a reproduction process. To comply with this, in a digital VTR or the like, a pilot signal is generated using a digital pilot tone. When such a pilot signal is added to a record signal, the error rate is not impaired.
As a record signal, the following record signals of three kinds, for example, are used. A first record signal is a code data F1 having a pilot signal of a frequency f1 and a notch at a frequency f2, a second record signal is a code data F2 having a pilot signal at a frequency f2 and a notch at a frequency f1, and a third record signal is a code data F0 having a notch at frequencies f1 and f2. These signals are recorded on a magnetic tape while switching them for each track.
In the track pattern, F0 is recorded on a first track, F1 on a second track, F0 on a third track, and F2 on a fourth track, and this sequence is repeated on the subsequent tracks. A tracking servo is realized by conducting a control in such a manner that, during the reproduction of, for example, F0 on the third track, the cross-talk levels of the pilot signals of f1 and f2 from the second and fourth tracks on both sides are equal to each other. According to this control, the tracking servo can correctly be conducted.
When a pilot signal is to be detected during a reproduction process, a band-pass filter (BPF) is used. Since tracks on both sides of a track containing a pilot signal have a notch at the pilot frequency, the S/N ratio of the pilot signal is increased, and that, even when the BPF has a small Q, the tracking properties are hardly affected.
FIG. 3 shows the configuration of an encoding apparatus. Numeral 49 denotes a parallel/serial converter which converts an input record data into a serial data. The parallel/serial converter 49 outputs the converted serial data to a 0 addition circuit 50, and a 1 addition circuit 51. The 0 addition circuit 50 adds 1 bit of "0" to the MSB of the record data and then outputs the data to a precoder 52. The 1 addition circuit 51 adds 1 bit of "1" to the MSB of the record data and then outputs the data to a precoder 53. The precoders 52 and 53 precode the input data and output the data to frequency component extractors 54 and 55, run-length detectors 56 and 57, and delay circuits 59 and 60. The frequency component extractors 54 and 55 extract the frequency components of the pilot frequency and the notch frequency, and output the extracted frequency components to an output determiner 58. The run-length detectors 56 and 57 detect the run length of the input data and output it to the output determiner 58. The output determiner 58 outputs a change-over signal to a switch 61 on the basis of the outputs of the frequency component extractors 54 and 55 and the run-length detectors 56 and 57. In response to the change-over signal, the switch 61 selects either of the outputs of the delay circuits 59 or 60, and output it as a code data.
Next, the operation of the encoding apparatus will be described. The parallel/serial converter 49 accumulates 24 bits of 8-bit record data, converts them into serial data, and outputs the converted data. When the bit frequency of a record data is indicated by fb, the read-out frequency fb' is obtained by an expression of fb'=(25/24).times.fb. The 1-bit addition is conducted by adding "0" or "1" (hereinafter, such a bit is referred to as "control bit") to the MSB of the record data by means of the 0 addition circuit 50 or the 1 addition circuit 51. The data to which a control bit is added is precoded by the precoders 52 and 53. The precoders 52 and 53 are of the I-NRZI type, and an EXOR of the input data and a 2-bit delayed data outputted from one of the precoders 52 or 53 is an output of the respective precoder 52 or 53.
The frequency component extractors 54 and 55 extract the frequency components of the pilot frequency and the notch frequency. When the code data to be generated is F1, for example, the pilot frequency is f1 and the notch frequency is f2. When the code data is F0, both f1 and f2 are notch frequencies and there is no pilot frequency. The run-length detectors 56 and 57 detect the run length of the input data. The delay circuits 59 and 60 delay the outputs only when the frequency component extractors 54 and 55 and the run-length detectors 56 and 57 operate.
The output determiner 58 controls the switch 61 so as to output a signal in which the pilot frequency component is larger and the notch frequency component is smaller, on the basis of the frequency components extracted by the frequency component extractors 54 and 55. When the run length is 10 or more, for example, the switch 61 is controlled so as to unconditionally output a signal in which the run length is shorter. When the state of the switch 61 is changed by the output determiner 58, the delay circuits 59 and 60 output data, and one of the data is outputted from the encoding apparatus as code data.
FIG. 4 shows the configuration of the prior art frequency component extractor 54 shown in FIG. 3. The configuration and operation of the frequency component extractor 55 are the same as those of the frequency component extractor 54, and hence the following description is conducted only on the frequency component extractor 54. The frequency component extractor 54 has adders 21, 30, 35, 40 and 45, holding circuits 22, 31, 36, 41 and 46, subtracters 23 and 26, a known DSV generator 24, squarers 25, 32, 37, 42 and 47, a known data generator 27, multipliers 28, 33, 38 and 43, a weighting adder 48, sine wave generators 62 and 64, and cosine wave generators 63 and 65.
The data from the precoder 52 is inputted to the adder 21 and the subtracter 26. The adder 21 adds the input value and a value held in the holding circuit 22, and the sum is held in the holding circuit, 22. The subtracter 23 obtains the difference between the DSV of an input signal which is an output from the holding circuit 22 and the known DSV generated by the known DSV generator 24, and the difference is output to the squarer 25. The squarer 25 squares the difference and outputs it to the weighting adder 48.
On the other hand, the subtracter 26 obtains the difference between the input data and a known data generated by the known data generator 27, and outputs the difference to the multipliers 28, 33, 38 and 43. The multiplier 28 multiplies a sine wave of the frequency f1 outputted from the sine wave generator 62 by the input data, and outputs the result to the adder 30. The adder 30 adds the input value and a value held in the holding circuit 31, and the sum is held in the holding circuit 31. The squarer 32 squares the value held by the holding circuit 31 and outputs it to the weighting adder 48. Similarly, the multiplier 33 (38, 43) multiplies a cosine wave of the frequency f1 (a sine wave of the frequency f2, a cosine wave of the frequency f2) outputted from the cosine wave generator 63 (the sine wave generator 64, the cosine wave generator 65) by the input data, and outputs the result to the adder 35 (40, 45). The adder 35 (40, 45) adds the input value and a value held in the holding circuit 36 (41, 46), and the sum is held in the holding circuit 36 (41, 46). The squarer 37 (42, 47) squares the value held by the holding circuit 36 (41, 46) and outputs it to the weighting adder 48.
Next, the operation will be described. A digital signal of "0" or "1" from the precoder 52 is inputted to the frequency component extractor 54. In the following description of calculations of the frequency components, however, the input of "0" is dealt as an input of a waveform of "-1". The frequency component extractor 54 extracts the levels of DC component, pilot component, and notch.
First, the method of extracting DC and pilot components will be described. The adder 21 adds the input value of "-1" or "1" and the value of the holding circuit 22, and the result is held in the holding circuit 22, thereby calculating the DSV. When the DSV is converged to the vicinity of 0, the DC component is eliminated. When the DSV is periodically varied, furthermore, the pilot component can be generated. In the following, a case where a pilot signal of the frequency f1 is generated will be described as an example.
The known DSV generator 24 generates a DSV (known DSV) which has a period of the frequency f1 and is a rectangular wave. The subtracter 23 calculates the difference between the DSV of the input signal and the known DSV. When the state of the switch 61 in FIG. 3 is changed so as to output a signal from which a smaller difference is obtained, it is possible to generate a signal which has no DC component and includes a pilot signal of the frequency f1.
Next, the method of extracting a notch component will be described. In the example, notch components of the frequencies f1 and f2 are extracted. In the case where the notch frequencies include a pilot signal, the pilot component is previously subtracted from the input signal so that a notch is generated in the vicinity of the pilot signal. When notch components of a signal including a pilot signal of the frequency f1 are to be extracted, for example, the known data generator 27 generates a data (known data) of "-1" or "1" having a DSV, the period of which is identical with that of the known DSV, and the subtracter 26 obtains a difference between the input signal and the known data.
Then the multiplier 28 multiplies sin.omega..sub.1 t it outputted from the sine wave generator 62 by the input data, the adder 30 adds the multiplication result to the value of the holding circuit 31, and the addition result is held. The squarer 32 squares the held value. The same operations are then conducted on cos.omega..sub.1 t, sin.omega..sub.2 t, and cos.omega..sub.2 t. Specifically, the multiplier 33 (38, 43) multiplies cos.omega..sub.1 t (sin.omega..sub.2 t, cos.omega..sub.2 t) outputted from the cosine wave generator 63 (the sine wave generator 64, the cosine wave generator 65) by the input data. The adder 35 (40, 45) adds the multiplication result to the value of the holding circuit 36 (41, 46), and the addition result is held. The squarer 37 (42, 47) squares the held value. In the same manner as the case where frequency components are extracted by means of the Fourier transform and the output is selected, the output determiner 58 of FIG. 3 compares the sums of the square results with each other, and the state of the switch 61 is changed so as to output a signal from which a smaller sum is obtained. As a result, a notch, and a notch in the vicinity of the pilot signal are generated in the frequency spectrum of the code data.
The weighting adder 48 conducts a weighted addition on the calculation results of the DC and pilot components and those of the notch components, whereby the level ratio of the components can be changed. When the pilot component is to be increased, the weighting factor for the calculation results of the pilot component is increased.
The 8-10 conversion which is a prior art record modulation system is conducted as described above, and has a low conversion efficiency of 80%. A conversion efficiency is used as an index for attaining a high density recording in a digital magnetic recording/reproduction apparatus. In order to attain a higher density recording, consequently, the conversion efficiency has to be improved.
When such a prior art encoding apparatus is used, a notch appears in the code data, and therefore the Q of a BPF of a pilot signal detector of the tracking servo system using a digital pilot tone can be reduced, thereby producing advantages that a tracking servo is stabilized and a reproduction circuit can be manufactured at a low cost. If the width of a notch can be increased, these advantages can be further improved.